1. Field of the Invention
This invention relates to digitally-controlled tracking impedances and to standing wave ratio measuring instruments and, more particularly, to those instruments which indicate the standing wave ratio without the need for manual calibration to a reference power level.
2. Description of the Prior Art
The condition of an RF transmission system is indicated by the standing wave ratio (SWR) as determined by the impedance match between the antenna and the transmission line. As the transmitter power in an RF transmission system is varied, the magnitudes of the forward voltage and the reverse (or reflected) voltage are subject to vary. As the impedance match is altered, the magnitude of the reverse voltage is subject to vary. It would be desirable to have a meter which is capable of continuously measuring the standing wave ratio of an RF transmission system without the need for recalibration caused by changing power and/or impedance characteristics. Various analog approaches have been suggested to provide such a meter as disclosed, for example in "A Simple Computing SWR Meter" by D. L. Fayman, QST, July 1973, or in U.S. Pat. No. 3,683,274. A combination digital and analog approach has been proposed in U.S. Pat. No. 3,704,409 to produce a digital indication of the ratio of the reflected voltage to the forward voltage; however, such an indication is not readily useful as an indicator of the standing wave ratio.